The power line transmission technique uses an electrical power distribution network as a communication infrastructure to transport digital information. It superimposes on the low-frequency electrical current (typically 50 hertz (Hz) in Europe or 60 Hz in the United States) a low-energy signal at a higher frequency. This high-frequency signal propagates along the lines of an electrical installation (for example the electrical mains wiring of a home). When it is received by a PLT receiver (also known as a modem) of the electrical installation concerned, the analog signal can be decoded into a bit stream so that the data that it conveys can be reproduced on an appropriate device (for example a computer). This kind of modem also includes means for processing redundant information (error corrector code) that makes the signal more robust to noise and attenuation.
This technology can therefore be used to produce a computer network in a building (office block, school, etc.). This is referred to as an “indoor” application.
Power line transmission can be effected at a high or low bit rate. Power line transmission at high bit rates uses OFDM (Orthogonal Frequency Division Multiplexing) multicarrier modulation in a frequency band that generally extends from 1.6 megahertz (MHz) to 30 MHz. Power line transmission at low frequencies generally uses frequencies from 3 kilohertz (kHz) to 148 kHz.
The electrical mains wiring of a domestic (or more generally “indoor”) installation constitutes a particularly difficult transmission medium for power line transmission. The inventors of this patent application have found that the transfer function of the transmission channel consisting of the electrical cable situated between two socket outlets of an electrical installation varies in a somewhat unpredictable manner. The transmission channel therefore has a phase and amplitude response that varies very considerably as a function of frequency. Measurements have demonstrated oscillations in amplitude response between 0 and −80 decibels (dB).
These strong variations of the transfer function of the transmission channel are caused in particular by the quasi-random behavior of the various loads on the electrical installation concerned, for example domestic appliances connected to the electrical mains wiring of the home. The response of the transmission channel is also variable over time, as a function of the operating status of those electrical appliances (in standby mode or operating), and even the presence or absence of the plug of an electrical appliance in a given socket outlet.
As a consequence of these strong variations of the transfer function of the transmission channel, certain frequency bands are subject to severe fading, varying in time, which means that it is not possible to guarantee error-free transmission of signals at high bit rates.
A number of techniques are known for compensating transmission problems caused by imperfections of the transfer function of a channel. These techniques are mostly digital techniques and generally rely on compensating losses caused by the imperfections of the transmission channel.
A first of these techniques uses spectrum spreading to transmit the signal over a frequency band wider than the set of frequencies that constitutes it, and thereby reduces the harmful effect of some transmission channel fading. At the receiver, the original signal is recovered by correlation. However, that spectrum spreading technique has the drawback that it is effective only for transmission at low bit rates, and can therefore not be applied to power line transmission of signals at high bit rates.
A second known technique applies equalization by means of a tuned filter to remove the amplitude and phase distortion caused by the channel. However, that equalization technique has the drawback that it is particularly complicated.
There is also known from the patent document WO 01/69812 “Powerline Communications system, Powerline Communication Transmission Network and Powerline Communication Device” a PLT communication device comprising an adaptive impedance, the value of which adapts automatically to changes in transmission conditions. That kind of adaptive impedance reduces the harmful effect of fading in a power line transmission channel, either by impedance conversion or by shifting such fading into frequency bands that are not used to transmit data.
A drawback of that technique is that it requires real-time evaluation of the transmission conditions on the electrical installation, in order to adapt the impedance of the device. That kind of evaluation is effected by software in the PLT modem. That kind of technique is corrective, and does not prevent the introduction of interference into the electrical installation.
Those methods all rely on digital processing of signals in the PLT modem and they therefore require software intervention at the modem level, which can be a problem with PLT modems already installed in an electrical installation, where updating the software may be difficult.
There is therefore a need for a technique that, instead of compensating losses caused by imperfections of the power line transmission channel in an electrical installation, prevents, or at least reduces, the occurrence of those imperfections. To be more precise, there is a need for a technique for preventing the connected status of the socket outlets (device operating, device on standby, no device) degrading the transmission characteristics of the channel consisting of the domestic electrical mains wiring.